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The University of Tasmania (UTAS) is a public research university primarily located in Tasmania, Australia. Founded in 1890



Sorption and Dissipation of Clomazone, Metribuzin, and Pendimethalin in Tasmanian Cropping Soils  [2007]

Cumming, JP;

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The behaviour of the residual herbicides clomazone, metribuzin, and pendimethalin was studied in four diverse Tasmanian cropping soils. The aim of this research was to identify the key soil properties and soil/herbicide reactions influencing the concentration of bioavailable herbicide in soils, and to use this information to develop management recommendations for the Tasmanian cropping industries. The soil/herbicide assessments undertaken in the project included V2 life, adsorption and desorption in intact soil as well as various soil fractions. The factors that affect herbicide break down and bioavailability, and the role of soil moisture and soil micro flora on these fate processes, was also examined. Large differences in herbicide behaviour were measured between soil types, and hypotheses proposed to explain these differences. The data generated in the project are valuable for the development of management recommendations for the Tasmanian cropping industries and to provide more locally validated parameters for pesticide impact models. Herbicide sorption and desorption for clomazone, metribuzin and pendimethalin varied according to soil type. The desorption constants (Kfd) calculated for clomazone and metribuzin exceeded the calculated adsorption constants (Kra) indicating some irreversibility of sorption. Correlations of sorption data with literature values and herbicide molecular properties also indicated that hydrophobic sorption to organic surfaces
could explain a large amount, but not all, of the observed sorption and desorption. Therefore, the complex relationship between mineral and organic surfaces was concluded to be having a considerable influence on sorption in the soils examined. The current reliance on experimentally derived Kra values for the modeling of herbicide behaviour was questioned based on the calculated experimentally derived values in this study. In particular, given the large differences between Kra and Krct values generated for the polar compounds clomazone and metribuzin, the use of Krct values may provide a better guide to long term bioavailability of applied herbicides for fate modeling. Rates of herbicide dissipation under field conditions were measured over 12 months at three locations. Dissipation was best described by the Hoerl equation, which provided a better fit to the data than the 1st order equation. Calculated half-lives from the 1st order equation were generally higher than reported literature values for clomazone, metribuzin, and pendimethalin. However, calculated first 50% concentration dissipation values (DT 50) using the Hoerl equation gave significantly lower values which were more in the range of previously reported half-lives. This indicates the 1st order equation is either overestimating half-lives or that they are longer under Tasmanian conditions. The herbicide half-lives were longer in the sandy soils used in the study, and reflected a risk of carryover injury in the soils given low rainfall and a high percentage of bioavailable herbicide residue. It was also apparent that soil moisture had a strong influence on dissipation, possibly through the mediation of soil structure and desorption of herbicide. In particular, the large differences in dissipation and desorption between the clay soils (Ferrosol and Vertosol) and the sandy soils (Kurosol and Sodosol) suggested that desorption was having a controlling influence upon herbicide dissipation. Adsorption and desorption of herbicides was shown to be strongly influenced by the chemistry of organic matter associated with different soil mineral fractions. Fractionation of the soils revealed highly heterogeneous organic matter chemistry in soil mineral fractions. Particulate organic matter was identified as an important transient sorption surface for low polarity herbicide products in particular. In addition, charcoal was identified as a highly reactive surface for herbicide adsorption. Unique hydrophobic surfaces were also described in the Kurosol which have an apparent high adsorption affinity for the herbicides examined. The results from this study have been used to fine tune label recommendations for the use of clomazone (Command®) in Australia with specific recommendations warning against potential carryover injury in sandy soils with moderate organic carbon contents. The published sorption and half-life data has also been made available for use in decision support programs such as CSIRO Australia's Pesticide Impact Rating Index (PIRI) model.